Stingless bees (Apidae, Meliponini), which are close relatives to honey bees (Apidae, Apini)
and share with them a highly eusocial colonial organization (Michener, 2000), are remarkable for their habit of producing virgin queens throughout
the year (Engels and Imperatriz-Fonseca, 1990). Most of
these queens are not needed for a colony’s survival or division, however, and get executed
some time after their emergence (Imperatriz-Fonseca and Zucchi, 1995). Only little is known about the mechanism that releases the execution behavior
in workers or about how a particular virgin queen is chosen as a new queen. Recently, we found
that in Melipona beecheii a virgin queen’s conspicuous behavior, which
consists of rapidly running over the brood area, thereby making spinning movements and beating
her wings, triggers the worker attacks (Jarau et al., 2009). A chemical signal or cue potentially released by such queens seems not to
account for her execution. Virgin queens were never attacked as long as they sat motionless on
the comb and the number of aggressive worker attacks significantly decreased from 15 to zero
within 10 min when we made an active virgin queen “behavior-less” through freezing (for
details see Jarau et al., 2009). During our study, in
which we observed the virgin queens’ behavior and interactions with workers in small
observation boxes that contained a comb of approximately 200 cells and initially 25–30 adult
bees (the number of which increased as further bees emerged from the cells), we registered one
interesting case of queen acceptance, on which we report in the following.

On the 21st day after we established an orphan condition in our observation boxes, we
observed a virgin queen (marked with color the first time we saw her) that was not
aggressively attacked by the workers, as is typical during queen elimination in M.
beecheii (Jarau et al., 2009). Instead, she
actively contacted the workers and bumped into them, often bringing herself into close lateral
contact with a worker and quickly shaking her abdomen in a wagging movement, thereby pushing
the workers away. The virgin queen’s aggressive behavior continued for the following three
days, but on the fourth day she slowly walked through the observation box without bumping into
workers. By that time she apparently was accepted by the workers and the observed calm
behavior continued until the end of our observations nine days after first seeing her. During
that time we observed only one further virgin queen, which was killed by the workers, and no
males emerged from the brood cells. Since the observation box was not connected to the outside
the accepted queen could not leave for a nuptial flight.

The virgin’s aggression towards the workers, especially the conspicuous abdomen shaking after
she brought herself into a position side to side with a worker, likely is an essential stage
in the establishment of her dominance by means of direct physical aggression. In accord with
our observation Silva et al. (1972) reported that a
M. quadrifasciata virgin queen began touching the vertex of workers with
her mandibles and forelegs on the third day after emergence and considered this behavior a
kind of dominance-subordination mechanism. Likewise, Engels and Imperatriz-Fonseca (1990) mention that in a variety of non-Melipona
species interactions between workers and virgin queens switch from an initial worker-
to queen control (without giving details, however). These observations are of interest because
the establishment of a single dominant female by means of overt aggression is regarded
characteristic of primitive insect societies (Wilson, 1971). We assume, that in highly eusocial stingless bees a virgin queen’s physical
aggression is important for the initial establishment of her dominance over the workers, too.
Once her dominance is established, however, she probably can be recognized by the workers by
her specific – maybe individual – cuticular hydrocarbon pattern. Her aggressiveness then
ceases, which finally may lead to the ritualized dominance behavior that is
typical for physogastric queens in stingless bees (Zucchi, 1993). In addition, virgin queens may produce pheromones – the composition of which
may change in time – that could lead to their acceptance. However, the existence of such
pheromones remains to be shown to this day.

The scenario described above is based on occasional observations only and should be
investigated in greater detail with a larger number of observed acceptances of virgin queens
in stingless bees. However, the proposed predominant role of a virgin queen’s behavior for the
establishment of her dominance over the workers is in line with the demonstrated importance of
her behavior during the queen elimination process (Jarau et al., 2009) as well as with the known importance of ritualized dominance behavior
of physogastric queens (Zucchi, 1993). Interestingly, in
these aspects stingless bees markedly differ from honey bees, where such agonistic behavioral
interactions with workers are lacking and where a queen’s identity and presence as well as her
dominance position are predominantly communicated by means of pheromones (Free, 1987; Winston, 1987).